The present invention relates generally to pulse wave receiver systems, and more particularly to a delay line canceler for filtering long pulse and continuous wave signals from being applied to a receiver designed for short pulse signal reception.
In pulse wave receivers it is often desirable to eliminate the effect of continuous wave (CW) signals, slowly changing phenomena such as clutter, and long pulse signals, such as pulsed Doppler signals, in receivers designed for short pulse signal reception. To eliminate these effects delay line cancelers are used to filter the unwanted signals from the signal source.
Dealy line cancelers work on the delay and substract principle as explained in Skolnick, "Introduction to Radar Systems," (1962) published by McGraw-Hill. Generally, a pulse wave signal is divided along separate transmission paths, the first path being a common transmission line and the second path being a delay line having a characteristic delay constant. The outputs of each path are then coupled to a differential amplifier, the first path coupled to the non-inverting input and the second path to the inverting input. For CW signals and pulse wave signals having a pulse width greater than the delay constant, the pulse wave signal will appear at each input to the differential amplifier. The differential amplifier is operative to subtract the voltage of the signal applied at its inverting input from the voltage of the signal applied to the non-inverting input. Since both voltages are equal, the differential amplifier will develop an output signal having zero voltage, effecting cancellation of the undesired signals. A typical delay line canceler as hereinabove described has been disclosed in an application entitled "Delay Line Cancellation Pulse Detector", Ser. No. 119,706, filed on Feb. 8 1980, and assigned to the assignee of the present invention.
The differential amplifier, being an active element present in the prior art delay line canceler, introduces temperature sensitivity into the pulse wave receiver system. This limitation arises from the temperature dependent common mode rejection of the differential amplifier inputs. Furthermore, the transmission line must be resistively tuned to the characteristic impedance of the delay line, which also is temperature dependent, for providing equal voltage at the inputs to the differential amplifier. These limitations limit the achievable suppression of undesired signals to about 40 db within a temperature range of -55.degree. C. to 95.degree. C.
A prior art delay line canceler as hereinabove described operates in frequency bands normally below 5.00 MHz. Circuit design techniques which rely on active elements introduce another limitation as the high cost of design or purchase of a microwave amplifier designed for the contemplated frequencies.